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Hi Tony,
In ISIS (and other amateur packages) this is usually taken account for in the single combined step for instrument and atmospheric response correction.
The approach is take a spectrum of a standard A or B type star that has a spectrum in the Miles or similar database, and that it is at a similar altitude to your target. You use your spectrum along with the one from the Miles database to calculate a combined instrumental and atmospheric response correction. Where this won’t work is if you try to observe a target at a significantly different altitude, or if the sky conditions change. In that case you would need to take a fresh spectrum of an appropriate A or B type star.
When removing individual atmospheric telluric lines then it will be necessary to take a more sophisticated approach. ISIS has another tool which can be used for that but often this step is not needed for the target spectra. I also find this only approximately works rather than giving a really good telluric removal.
Cheers,
Andy
Hi Alun,
As David says, this spectrum would be a welcome addition the BAA Spectroscopy Database, and expect you have others.
The spectra need to be in 1D fits format with the BeSS fields populated. I know BASS can produce this format, so just let me know if you need any pointers. Otherwise just email me a sample 1D fits file when you are ready and I will use that to set you up in the database. I don’t put my personal email address online, but you can email me at vssdbm <at> britastro.org
Cheers,
Andy
That is a nice image for 400 seconds. I will be interested to hear how you get on with that project.
Cheers,
Andy
Hi Alun,
M87 could be a very interesting target. I have no idea whether the features, like the plume or jet, are within reach of our spectroscopes. If they are then the results would be fascinating. The central black hole might show broad emission lines due to fast rotating gas, while a jet might show a big blueshift.
Cheers,
Andy
Hi Alun,
Your reminded me of when I took a spectra of M81 and M82. Like you I was just interested to see what I could see in spectra of galaxies. My setup is for high resolution spectra, so it is very hard to go faint and I just see a small region of the spectrum. M81 was almost entirely noise, while M82 though noisy did have some interesting features I could investigate.
Below are the spectrum and the view through my guide camera. It turns out I had placed the slit on some known HII regions called regions A and D. This allowed me to do a radial velocity calculation that showed a redshift of about 255 km/s.
It is nice to sometimes take a spectrum of an object without preconceptions. Then you can experience the joy of discovery, even if many others have been there before.
Cheers,
Andy
Hi Peter,
This is a fantastic discovery, a system with 7 Earth sized planets! I expect we will learn some of the real science behind this discovery and the future research of this system at the Winchester Weekend. Prof Didier Queloz is giving the Alfred Curtis Memorial Lecture, and he is one of the authors of the Nature paper announcing this latest discovery. Brilliant timing by the meeting organiser!
I agree the media tends to get carried away, and social media even more so. It is a shame as I think the hype can miss the excitement that we do not know everything yet. So there will be lots of discoveries along the way as we learn about this planetary system and others which are no doubt waiting to be discovered.
Cheers,
Andy
Hi Alun,
That is an interesting spectrum. The hydrogen emission lines that you have picked up will be from HII regions within NGC 2903. Doing a little digging around I see that this galaxy has a high star formation rate. So it makes sense that it has a lot of bright clouds of hydrogen which will be forming stars.
Best wishes,
Andy
Hi Tony,
I’ve submitted a few observations to BeSS.
The good news is that the file format is identical to the BAA Spectroscopy Database format. They in fact came up with the format, and we used it as it makes sense to have one format across the different databases.
You can download spectra from the BeSS website, and on the registration screen there is a tool you can use to check your file.
http://basebe.obspm.fr/basebe/Accueil.php
The administrators are very friendly and knowledgeable, but note they can sometimes take a few weeks to respond.
Cheers,
Andy
Hi Paul,
That sounds like it would be an excellent setup for variable star observing. Some BAA observers have started to use Muniwin for variable star photometry as it produces output files in the format for upload into the BAA VSS database.
http://www.softpedia.com/get/Science-CAD/MuniWin.shtml
As well as looking at the VSS website it is worth dropping Roger an email line if you would like to start variable star observing. The section is very active and we are always keen for new members. Of course this forum is also ideal for any questions as there are other people can look at the answers.
Best wishes,
Andy (BAA VSS Database Secretary)
Hi Jack,
That error message indicates that crucial FITS header fields are missing. First out I would double check that it is definitely a FITS file which MaxIm DL created, as I know it can save multiple file types. Otherwise I think something odd that has gone on with your bias, dark and flat processing.
Cheers,
Andy
Hi Andrew,
Please excuse my long posts 🙂
It is good to have these discussions as I am sure that others have similar questions.
Best wishes,
Andy
Hi Andrew,
You raise some important and interesting points, possibly worth their own forum threads.
I agree the processing of a spectrum should depend on the scientific use to which it is intended to be used. When submitting to databases though, it is my opinion that it is best not to rectify the spectrum unless necessary. The reason for this is that a researcher can always rectify a spectrum should they wish to do so, though they cannot unrectify it. Note that when a rectified spectrum is submitted to the BAA Spectroscopy Database then the BSS_NORM fits header should be populated.
I have no strong opinion on whether or not to rectify a spectrum when no response correction has been applied.
On the dark vs bias front, I would expect to see differences between dark and bias frames with amateur CCDs. I also use an SXVR-H694 and I see a big difference between my bias and dark frames.
For the following stats I had the CCD set to -20C with 600 second darks, with statistics computed by MaxIm DL from single raw bias and dark frames.
Bias Dark
Mininum 882 1006
Maximum 1315 56522 (due to hot pixels not cosmic ray hits)
Average 1084.580 1133.068
Standard Deviation 15.960 211.396I also advise to flat field as know of a couple of reasons why the response correction from a reference star would not correct for all variations in the CCD.
First, when using a reference star to create an instrumental and atmospheric response curve, then there is a degree of smoothing to remove small scale variations in the curve. This is to give the bulk wavelength dependency. So by its very nature it does not remove the individual pixel to pixel variations.
Second, the target star and reference star on the CCD need to be exactly aligned so that the light path through the spectrograph illuminates the CCD pixels in exactly the same places. This is nearly impossible with a slit spectrograph, but I am not sure if a fibre fed system would get around this.
I am happy to be proven wrong if my understanding is at fault. Also please excuse this lengthy post but I thought it worthwhile for these important topics.
Kind regards,
Andy
Hi Kevin,
I’ve been doing spectroscopy for a couple of years now and I still get excited with each new spectrum. You never know what you will see and this is very true of Be stars. The hydrogen lines can be a complex mix of absorption and emission from the star and disc configurations which change over time.
I found 48 Per in the BeSS database. I’ve put a link below, though it does not look as though the link preserves all of the criteria I entered. You can select the H-beta region by entering 4861 in the wavelength. This shows pervious spectra with various mixes of absorption and emission of this line.
http://arasbeam.free.fr/spip.php?page=beam_splist2&etoile=HD 25940&lang=en
I’m not sure of the effect of rectification on the science value of a spectrum, so I avoid it. The BeSS standard uses the words ‘clearly advised against’ where it calls it ‘continuum normalisation’. For personal interest it is of course fine, and others with more knowledge may be able to advise on its effect. The bit where I am particularly unsure is its effect on line strength measurements, like equivalent width. This may not be big since the effects to the continuum on either side will be scaled by a similar amount, but I’ve never done an experimentation to discover the effect.
Similarly, for personal interest it is fine to use alternative methods to correct the response, but as soon as you want to do any science or submit to a database then a proper response correction is needed.
Best wishes,
Andy
Hi Kevin,
Nice spectrum!
I check, or average out, mechanical shifts in my spectrograph by taking neon calibration frames at the start and end of each stellar spectra. However, I’m using an LHIRES III which is more prone to flexure than the Alpy, plus I frequently stay on the same star for an hour or more. So you may not need to do this for your setup.
When checking a radial velocity, then ISIS has a useful tool in the “Misc” tab for calculating the heliocentric velocity correction, so the rotation of the Earth and its orbit around the Sun (bigger contribution) can be calculated and removed. Then I find SIMBAD is an excellent resource for checking the expected radial velocity of the star.
http://simbad.u-strasbg.fr/simbad/
Best wishes,
Andy
Hi Jack,
The error message or screen shot did not get included in your post.
In general terms, if BASS cannot open it then that implies something went wrong with the calibration process. Before moving to ISIS I carried out bias, dark and flat calibration in MaxImDL, followed by spectrum extraction and wavelength calibration in BASS.
Best wishes,
Andy
Hi Jack,
I would just re-emphasize that the author of BASS is re-writing the calibration part of the software. So expect changes to BASS calibration, and I’m not sure where the deficiencies currently lie in the BASS calibration routines. That is why I was suggesting MaxImDL. Once John has finished with the BASS enhancements I am hopeful that will contain useful features.
I usually take a minimum of 20 calibration frames. This reduces the noise when they are combined, which is an unavoidable element of calibration frames.
-5C is OK but if you can go colder then that will reduce the noise in your spectra. Just emphasizing again that the temperature of your CCD when taking the calibration frames must match the temperature when acquiring spectra. So you might need one set of calibration frames at -5C for your older spectra. Then another set at a cooler temperature if your CCD can go colder and hold a stable temperature to within at most a few tenths of a C.
Best wishes,
Andy
Hi Jack,
I think Robin and Paul have covered this off well, but I’ll add and re-iterate a few points. It is worth noting that this is the same process as is used for deep sky imaging. The main difference is that you can’t use “sky flats” or just any kind of flat lamp. Instead you need a flat lamp that produces a nice smooth continuous spectrum. The LHIRESIII, ALPY, and LISA all have such a flat lamp built in.
I’ll add that your calibration frames need to be taken at the same CCD temperature, and this should be as cold as possible, whilst ensuring that your CCD can easily maintain this temperature. I find that my SXVR-H694 can hold -20C during the Summer as well as the Winter months.
Best practice is to take of the order of 20+ frames of each type. I was actually taking a new set of darks and bias frames last night. Long overdue as over time the performance of a CCD can change. I try to retake darks and bias frames every 6-12 months. Flats I retake about once a month if I don’t touch the spectrograph. If I change anything on the spectrograph then the flats have to be retaken. This includes focusing the spectrograph collimator lens, but not refocusing the telescope.
I know that John Paraskeva is writing some new calibration routines for BASS. Though MaxImDL has perfectly good calibration routines which work fine for spectroscopy.
Best wishes,
Andy
Hi Jack,
First, just to note that PTAREA is not one of the FITS header requirements for BeSS, in case this is why you are asking.
To calculate the area you use the formula for the area of a circle.
Area = pi x radius squared
The Celestron website gives the diameter of your C14 as 355.6mm, so the radius is 177.8mm. Thus the area would be:
3.14159 x 177.82 = 99315mm2
This does not take into account is the central obstruction in your C14. Again the Celestron website gives this as 114mm, so a radius of 57mm. Thus the area of the central obstruction would be:
3.14159 x 572 = 10207mm2
So the actual light collecting area of your C14 will be:
99315 – 10207 = 89108mm2
Cheers,
Andy
Hi Kevin,
I find this depends on the field and the brightness of the star. I typically guide on another star in the field and there are usually several candidates, with my guide images of 5 to 10 seconds. However, this may depend on how good your mount is at tracking.
I use PHD2 and have guided on the target star on occasion. I’ve not found any problem with this, but there does need to be a strong signal outside of the slit. I’ve not looked in any detail as to how this works, whether it needs bimodal guiding or not. If the star is bright enough then I tend to notice it dimming rather than disappearing or splitting in two. This is usually the case when I am taking a spectra of a bright star and so the fainter stars are not easily visible in the guide image.
Cheers,
Andy
Hi Kevin,
For locating targets when polar alignment may be a bit off, I suggest star hopping. Start with a bright star that cannot be mistaken for anything else, then move towards the target star, if necessary locating and centering some other stars of intermediate brightness along the way.
The guide image in spectroscopes tends to be of lower quality due to the transfer optics, but it is a lot better than it used to be. I find that I can sometimes solve my guide images with Astrometry.net, but not always. I’ve also taken to storing a guide image, so that I can always go back and check if the spectrum turns out to be unexpected. Observing variable stars can be a bit easier as there will often be a BAA chart and/or and AAVSO chart. With these I find it straightforward to confirm that I’ve got the right star.
That is a great first spectrum. I agree something does not look right with the continuum, but it can take some trial and error to get the response profile working well. Robin’s suggestion of observing 2 Miles stars, and using one to create the response profile to correct the continuum on the other is very good. Especially if you choose A or B type stars which have simple spectra so it is easier to see what is going on with the response profile. The correspondence between the features in the spectra shows that your wavelength calibration is working well.
Best wishes,
Andy
